The present invention relates to adsorbents for removing impurities from water-comprising gas streams, in particular for use in fuel cell systems.
Natural gas and biogas are attractive hydrocarbonaceous gases which are very highly suitable for producing hydrogen-comprising gases for, for example, downstream fuel cell applications. Numerous processes are known for obtaining hydrogen from hydrocarbonaceous gases. Natural gas and also biogas, in addition to hydrocarbonaceous constituents, comprise numerous impurities such as sulfur-, silicon- and halogen-comprising compounds which are found at differing concentrations. In addition, further sulfur compounds must be added as odorants to commercial natural gas, which odorants are distinguished by a strong odor, in order to make unwanted gas escapes, for example, noticeable.
All known processes for producing a hydrogen-comprising gas stream have in common the fact that production thereof comprises at least one process stage in which a catalyst is used. All catalysts known therefor have the disadvantage of being poisoned, that is to say irreversibly deactivated, by sulfur compounds which are present in the gas streams. The poisoning leads to drastically lowered service lives and in the extreme case even to complete failure of the catalyst.
For economic operation of the recovery of hydrogen-comprising gases from hydrocarbonaceous gas streams it is therefore necessary to remove in particular the sulfur compounds present in the hydrocarbonaceous gases.
A general process for purifying gas streams is the adsorption of higher-molecular-weight constituents to chemical adsorbents or oxidizing agents, such as activated carbons or molecular sieves.
However, all these processes, depending on the operating conditions, suffer from disadvantages:
Physically acting absorbents, in the removal of sulfur compounds, also permit a substantial drying and removal of higher hydrocarbons. However, the processes only operate satisfactorily at correspondingly higher pressures.
In the case of processes which operate using chemically acting absorbents, pressure and temperature are not of prime importance. Usually, use is made of aqueous solvents such as monoethanolamine or diethanolamine in an order of magnitude of 2.5 to 5 n solutions, using which sulfur compounds are preferentially removed. The gases, after the treatment, are usually saturated with water vapor.
The known iron oxide and iron gelate processes use mostly iron gelate compounds, at which hydrogen sulfide is preferably converted to iron sulfide and water or elementary sulfur. The gases are in turn saturated with water vapor. Organic sulfur compounds are scarcely removed or not removed.
In the case of adsorption processes using solid adsorbents, use is made of adsorbents such as activated carbon, molecular sieves, carbon molecular sieves, silica gel, KC-Perlen, or mixtures thereof which, even in the presence of carbon dioxide, enable desulfurization with simultaneous removal of water vapor. However, the formation of COS from hydrogen sulfide and carbon dioxide is frequently observed as a side reaction.
In the case of the processes which operate using solid adsorbents, usually the raw gas stream is passed through an adsorber in which the solid adsorbent is situated. In the adsorbent bed, water vapor is adsorbed in the lower layer of the bed and the hydrocarbons are adsorbed in the upper layers. In the case of a possible regeneration, a substream of the raw gas is branched off, passed over a heater and heated to regeneration temperature. In the second adsorber the previously adsorbed hydrocarbons and water vapor are expelled. The departing desorption gas is subsequently passed over a cooler in which the adsorbed hydrocarbons and water vapor condense and are separated off in a downstream separator. The regeneration gas is then, after throttling the raw gas stream, added back to the raw gas stream which is passed over the adsorber which is being loaded and in which water and hydrocarbons are again adsorbed.
Depending on the site of origin and treatment, these gas streams still comprise differing concentrations of water. The concentration can also vary further as a result of external factors such as temperature and atmospheric humidity. Water, however, depending on the pore system of an adsorbent, leads to pore condensation. That is to say even far removed from the dew point, condensation of water can occur owing to capillary forces in the pore system of an adsorber. The water layer then prevents the contacting of impurity and active center on the adsorbent, and so its adsorption capacity falls drastically. In this case between 5 ppm up to several percentage points of water can be present in the gas stream.
The problems of sulfur removal in fuel cell systems are described extensively in the publication BWK 54 (2002) No. 9, pages 62-68. In this publication, again reference is made to the lack of a simple solution for removing all of the sulfur components from natural gas.
WO 2004/056949 introduces adsorbents for removing sulfur components from gases. In this publication, explicit consideration is given to non-zeolitic systems, the active component of which in most cases is Cu. These systems, as may be found in the examples, may readily be used in the purification of dry natural gas. Their use in water-comprising gases (moist natural gas, biogas or the like), however, is problematic, since the water which occurs here condenses in the small pores of the adsorbents and makes them inaccessible to chemisorption or physisorption.
TDA Research Inc. mentions in a publication (Am. Chem. Soc., Div. Fuel Chem. 50(2) (2005) 556ff.) the problems of removing sulfur components from water-comprising gas streams. The publication also shows a comparison with activated carbons (Norit RGM3 Activated Carbon) and zeolites (Grace). The SulfaTrap™ material introduced by TDA shows a capacity of less than 0.01 kgs·lcat−1 and thereby not an approximately economic performance.
The problems in removing impurities from water-comprising gases are also described in DE-A-100 34 941. However, here, a processing solution for the problem is sought which plans for uneconomic residence times on the adsorbent and the additional use of molecular sieves, etc., for water removal.
Zeolitic systems, owing to their pore size, have a tendency to extreme uptake of water, in such a manner that their capacity for sulfur components is significantly decreased.
In the purification of biogases, a switch must frequently be made to significantly increased temperatures, in order to prevent pore condensation. The use of ZnO in such an application at T>300° C. is demonstrated in Energy&Fuels 18 (2004) 576ff. or else U.S. Pat. No. 4,871,710, example 1 (T=150° C.).